Growth of uniform crystals

ABSTRACT

The invention provides for growing semiconductor and other crystals by loading a vessel in its lower portion with a seed crystal, loading a charge thereon in the vessel, heating the charge to a molten state and electromagnetically stirring the melt using magnetic and electric fields to obtain a more uniform composition of melt and slowly reducing the temperature of the melt over the crystal to grow a more uniform crystal from such stirred melt.

RELATED APPLICATIONS

This application relates to provisional application 60/285,914, filed 24Apr. 01, from which domestic priority is claimed.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

FIELD OF THE INVENTION

This invention relates to controlled growth of crystals, particularlyfor growth of more uniform crystals.

BACKGROUND OF THE INVENTION

The favored research approach to achieve ultra high-quality compoundsemiconductor crystals is the bottom-seeded method—either the VerticalGradient Freeze (VGF) or Bridgman technique. There are clear advantagesof these methods over the more common top-seeded Czochralski (CZ)method, However, in some cases the crystal growth rate can be severelylimited and in others the composition of the crystal can be non-uniform.

In the above bottom-seeded method, seed crystal 2 is positioned undergrowth crystal 4, in turn under melt 6, in vessel 8 as shown in FIG. 1.

In the VGF and Bridgman methods of crystal growth (FIG. 1),solidification is initiated either from a seed or from spontaneousnuclei at the bottom of a molten charge as opposed to the CZ method inwhich a seed is dipped into the melt from the top. Convective stirringdue to thermally driven buoyancy, that is found in CZ melts, is notpresent in bottom-seeded melts, where the thermal gradient increasingfrom the bottom to the top of the melt provides thermal stability. Thisabsence of strong convection, in fact, provides some of the advantagesof the bottom-seeded methods over the CZ method (fewer dislocations andlower twinning probability). However, the absence of melt convectionalso generates the melt condition that limits the growth rate and causescompositional non-uniformity.

Generally the chemical composition of a solid is not precisely the sameas that of the melt from which it freezes. This is known asnon-congruent melting, as opposed to congruent melting, in which thecomposition of the solid and liquid phases are identical. For alloycrystals such as Ga_(1-x)Al_(x)Sb, for example, the solid compositioncan be very different from that of the melt. Therefore duringsolidification one, or more of the constituent elements is rejected intothe melt to form a boundary layer of liquid with a chemical make-up thatis different from that of the bulk of the melt. This boundary layerbuilds up just adjacent to the crystal-melt interface. Strong convectivestirring due to thermally driven buoyancy, is not present inbottom-seeded melts because the top is hotter than the bottom. Thereforethe most effective transport mechanism in the boundary layer of a VGFmelt is diffusion, which tends to be quite slow. This slow rate ofdiffusion determines the crystal growth rate limit. Typical growth ratesfor bottom-seeded melts are nearly an order of magnitude less than thosefor top-seeded CZ growth and therefore the cost of producing VGFcrystals is greater. In addition, if there is virtually no mixing in themelt, the composition of the grown crystal can exhibit radialnon-uniformity if the melt composition is not initially uniform from thecenter to the periphery.

Accordingly there is need and market for an improved growth process forthe above alloy crystals that overcomes the above prior artshortcomings.

There has now been discovered a process for more controlled crystalgrowth to obtain more uniform crystals; both alloy crystals andnon-congruently melting binary crystals.

SUMMARY OF THE INVENTION

Broadly, the present invention provides a method for growing a moreuniform crystal by bottom seeding which includes,

-   -   a) loading a vessel, in its lower portion with a seed crystal,    -   b) adding a charge thereon in said vessel,    -   c) heating the charge to a molten state to form a melt,    -   d) electromagnetically stirring the melt to promote uniformity        over the crystal and    -   e) slowly reducing the temperature of the melt over such crystal        to grow the latter.

The invention also provides more uniform crystals as grown by the aboveprocess;

-   -   Examples of crystal growth methods with electromagnetic stirring        are found in the prior art, e.g., in U.S. Pat. No. 5,769,944 to        Park et al (1998) and in a Paper entitled Silicon Crystal Growth        by the Electromagnetic Czochralski (EMCZ) Method, Jpn. J. Appl.        Phys. Vol. 38 (1999) pp. L10-L13, by M. Watanabe et al. which,        however, relate to the CZ or top seeding method of crystal        growth, with the attendant problems of non-uniform composition        and higher defect density due to convection and the steep        temperature gradient. Thus the prior art above does not provide        for forming a uniform alloy crystal.

Definitions:

By “precursor charge”, as used herein, is meant a charge that can beheated to a melt that can grow, upon cooling, into a crystal of desiredcomposition

By “crystal”, as used herein, is meant a sizeable (up to 2-inch diameteror more) solid body, with the same crystallographic structure andorientation.

By “slowly reducing the temperature of the melt”, as used herein, ismeant to decrease linearly with time the temperature of the liquid abovethe growing crystal by reducing the heat emanating from various heaters.

By “vessel”, as used herein, is meant a container, a crucible of carbon,of quartz, of boron nitride or any other material that will not reactwith the melt.

By “alloy”, as used herein, is meant a mixture or combination of two ormore constituents in a crystal whose melting temperature andcrystallographic properties are determined by its composition.

By “dopant”, as used herein, is meant an additional minor element mixedwith the major constituents, which alters some crystal properties, butdoes not significantly alter the melting temperature.

By “uniform”, as used herein, is meant having a constant composition(alloy or dopant) throughout the crystal body or nearly so.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more apparent from the following detailedspecification and drawings in which;

FIG. 1 is a perspective, schematic view of a vessel for un-enhancedcrystal growth per the prior art;

FIG. 2 is a perspective schematic view of an embodiment for enhancedcrystal growth per the present invention;

FIG. 3 is an isometric vector diagram relative to FIG. 2;

FIG. 4 is a perspective schematic view of another embodiment of theinvention and

FIGS. 5 & 6 are elevation schematic views, partly in section, of yetanother embodiment of the invention for enhanced crystal growth.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring in more detail to the drawings, the present invention utilizesmagnetic and electric fields to provide stirring during the growth ofsemiconductor crystals from bottom-seeded melts. One procedure forgrowing a crystal with such stirring, is to load a cylindrical crystalgrowth crucible 11 that is closed at the bottom and open at the top witha single seed crystal 12 at the bottom and a charge of one or morechunks or particles of a solid (at room temperature) on top of it (notshown). The so loaded crucible is then heated by a helical coil 22, sothat the temperature increases from bottom to top of the charge, untilall of the charge and a small portion of the seed 12, become molten toform melt 14, as indicated in FIG. 2.

A small-diameter electrode 16 made of electrically conducting materialthat does not chemically react with the molten charge is insertedvertically into the center of the melt so that it extends to a pointjust above the seed crystal 12 but does not touch it. If the crucible iselectrically conducting, it serves as the second electrode 17 and avoltage source 18 is connected between the two. In this case, if themelt 14 is a good conductor, an electric current will flow radially from(or to) the center electrode 16. A magnetic field B (per FIG. 3) isinduced in the melt in the vertical direction by a magnetic coil orsolenoid 21, as indicated in FIG. 2. The magnetic field B, perpendicularto the current I, will induce a tangential force F (per FIG. 3) andunder many conditions, create a tangential flow 20 (per FIG. 2) andcause mixing in the melt.

Alternatively the coil 22 can be the same one that serves as a heatsource around the vessel. A magnetic field of about 50 gauss generateddirectly with a heating coil together with a current of several tens ofamperes can be sufficient.

In one example, it has been demonstrated that for a two inch diametermelt of GaSb, about 5 amperes of current together with a magnetic fieldof 50 gauss produce melt rotation rates of about 15 RPM.

To grow the crystal, the temperature is slowly reduced so that the meltis progressively crystallized onto the seed. As the height of thecrystal interface 23 increases, the electrode 16 is growth interface,e.g., 23 to 24, by utilizing an electrical insulator 15 over the upperlength of the center electrode, per FIG. 2.

Also per FIG. 4, if the crucible 13 is not electrically conducting, athin-walled cylinder 26 of conducting material or one, two or more smalldiameter electrodes (not shown, but similar in size to electrode 19 inFIG. 4) near the periphery of the melt can be deployed within thevessel. If the melt is volatile, an inert encapsulant can be employedand the growth takes place in a high-pressure chamber (not shown).

Electromagnetic stirring can also be employed to improve the quality andincrease the growth rate of crystals grown by a bottom-seeded techniquein which a submerged heater 32, in vessel 30, per FIGS. 5 & 6, isemployed to promote good long-range chemical uniformity. A schematiccross-sectional elevation of the crystal growth apparatus 30 is shown inFIGS. 5 & 6. The submerged heater 32, in its housing 52, effectivelyisolates two melt zones (upper and lower) during growth, to provide aconstant alloy composition in the lower zone and thus also in thecrystal to be formed. The system utilizes coil (34) and planarresistance heating elements (36) as shown. A fused silica crucible 38contains a large-diameter hollow-core upper charge to supply melt 40 anda smaller diameter lower charge to supply melt 42 as well as afull-diameter seed crystal 44. The graphite electrodes (including thesmall diameter electrode 46 in the center, and the outer electrode 48surrounding the growing crystal) are used to pass the radial currentwhich stirs the lower melt 42 when a vertical magnetic field is alsoapplied in the manner of FIGS. 2 & 3 above. As shown in FIG. 5, a copperwire coil 50, wrapped on the outside of a lower insulating tube (notshown) provides the magnetic field. At the beginning of a growth run,the upper and lower charges are made molten or melts 40 & 42, byapplying power to the heaters; i.e., the coil “side heaters” 34, the“submerged heater 32”, and the “external lower heater 36”. A smallfraction of the seed 44 is also melted just prior to growth. To grow thecrystal, the temperature of the BN disc 36, just below the seed 44, isramped down with a temperature controller (not shown) connected to theexternal lower heater. At the same time, the submerged heater housing 52is slowly raised, and the temperatures of the side heaters are rampeddown as well. As the crystal grows, per FIG. 6, molten material flowsfrom the upper melt 40 to the lower one 42, through small holes in theouter electrode tube and down through the annular space between theouter electrode 48 and the submerged heater housing 52, as indicated inFIGS. 5 & 6. The aim is to maintain a constant lower melt height of theorder of, e.g., 1 cm while the crystal grows upwardly from the seed. Adrawing of the system after some growth of crystal 56 has taken place,is shown in FIG. 6. Since the lower melt 42 is replenished with liquidfrom the upper melt 40 through the annular space 54, if there were nomixing, the liquid composition of the annulus 54 could be quitedifferent from that of the remainder of the lower melt 42. In this case,stirring the molten melt 42 below the submerged heater 32 with electricand magnetic fields can provide a more uniform radial composition bothin the melt 42 and in the crystal 56, in addition to permitting a morerapid growth rate.

Thus the present invention employs magnetic and electric fields toprovide stirring during crystal growth where uniform composition andhomogeneous properties are required. The lack of stirring is a chronicproblem for crystals that are grown from bottom-seeded melts. In thepresent invention, the melt is mixed by the Lorenz force arising when anaxial magnetic field is applied together with a radial electric current.This novel technique is applied to solve a chronic problem associatedwith many semiconductor crystals and alloy crystals, which can be ofgreat importance militarily or commercially. The problem has been that,in commercial practice, low defect density crystals can only be obtainedby bottom-seeded methods, but these methods are very slow and lacking inuniformity of crystal properties. The bottom-seeded method of thepresent invention overcomes the above disadvantages and there is a needfor crystals with low defect density and uniform properties such as GaSbfor IR transparent windows for IR imaging arrays. Such crystals do notexist at present. The method of the present invention offers thefollowing advantages:

-   -   1. Faster growth rates    -   2. Controlled crystal properties    -   3. Uniform alloy composition and doping concentration

The advantages of the electromagnetic stirring of bottom-seeded crystalgrowth melts are realized through the changes that stirring creates inthe boundary layer above the growing crystal. Since the crystal isfrozen from the liquid in this layer, its radial uniformity dependsdirectly on the chemical uniformity of such layer, and stirring canimprove it.

A value of this invention militarily is that new high-quality crystalsubstrates will become available for high-speed photonics and advancedmicro-electronic circuits. Prior to this invention, the majority ofsemiconductor crystals have been produced commercially by top seededgrowth methods, which have typically high defect densities and are notsuitable for advanced applications such as long wavelength IR lasers anddetector arrays.

A benefit of this invention is to improve the quality and reduce thecost of compound melt-grown bulk semiconductor crystals byelectromagnetic stirring. Such crystals are generally sliced into wafersand used as substrates for epitaxial growth or for ion implantation.These crystal wafers are the building blocks for structures that enablethe fabrication of virtually every electronic and optical system beingproduced or in development.

The type of crystals grown herein include crystals of alloy crystalssuch as Ga_(1-x)Al_(x)Sb, or In_(1-x)Ga_(x)P, for example or anycombination of mixed Group III and Group V elements of the periodictable. In general, any incongruently melting crystal material can begrown by this method.

1. A process for growing a more uniform crystal by bottom seedingcomprising, a) loading a vessel in its lower portion with a seedcrystal, b) adding a precursor charge thereon in said vessel, c) heatingsaid charge to a molten state, to form a melt, d) electromagneticallystirring said melt to form a more uniform composition melt over saidseed crystal and c) slowly reducing the temperature of said melt oversaid seed to grow said crystal.
 2. The process of claim 1 wherein avertical magnetic field is induced in said vessel and an electric fieldis applied orthogonally to said magnetic field in such a manner that arotational stirring of the melt results.
 3. A process for growing a moreuniform crystal by bottom seeding comprising. a) loading a vessel in itslower portion with a seed crystal; said vessel having electricallyconductive walls or if not, an annular sleeve that closely fits withinsaid walls, which sleeve is conductive to define an outer electrode, b)loading a charge on said seed crystal within said vessel, c) mounting anelongated inner electrode centrally, or nearly so, within said outerelectrode, so it extends to said charge but does not contact saidcrystal, d) positioning an inductance coil around said vessel, e)heating the so loaded charge to a molten state or melt, f) applying avoltage across said two electrodes so as to impose an electric currentin said melt and g) applying a voltage across said coil to induce amagnetic field in said melt to produce a stirring force therein toimprove the uniformity of melt and crystal.
 4. The process of claim 3wherein the vessel and its contents are heated in such a manner that thetemperature of said charge increases from bottom to top of said chargeso that all of the melt and a small portion of the seed become molten.5. The process of claim 3 wherein said inductance coil also serves toheat said charge to said molten state, which coil also serves togenerate said magnetic field and stirring forces in said melt.
 6. Theprocess of claim 3 wherein the temperature of melt in the vessel isslowly reduced to grow said crystal at the melt-crystal interface. 7.The process of claim 6 wherein said elongated electrode is raised inadvance of the rising crystal growth in said vessel.
 8. The process ofclaim 3 wherein when said vessel is not electrically conducting, a thinwall cylinder of conducting material or one or more small diameterelectrodes can be placed in the vessel near the periphery of the melt.9. A process for rowing a more uniform crystal by bottom seedingcomprising. a) loading a vessel in its lower portion with a seedcrystal, said vessel having electrically conductive walls or if not, anannular sleeve that closely fits within said walls, which sleeve isconductive, to define an outer electrode, b) loading a lower charge onsaid seed crystal within said vessel to supply a lower melt, c) loweringan inner heater in a heater housing in said vessel, onto said lowercharge, which heater housing is sized to leave one or more annularspaces between it and the vessel interior walls, d) mounting anelongated inner electrode centrally, or nearly so, within said outerelectrode, so it extends through said heater housing and into saidcharge but does not contact said seed crystal, e) loading an uppercharge into an upper reservoir of said vessel to supply an upper meltwhere it can flow down through said annular spaces and around saidheater housing to contact said lower melt and thus submerge a portion ofsaid housing in said melt, f) applying heat from the sides and frombelow said vessel and said seed crystal and from the inner heater abovethe melt, to render said upper and lower charges molten, to form saidupper and lower melts and to render a portion of said seed crystalmolten proximate said lower melt, g) positioning a solenoid around saidvessel, h) applying a voltage across said two electrodes so as to imposean electric current in said lower melt and i) applying a voltage acrosssaid solenoid to induce a magnetic field in said melt to produce astirring force therein to improve the uniformity of melt and crystal.10. The process of claim 9 further comprising, j) ramping down thetemperature below said vessel and seed crystal and slowly raising saidinner heater and its heater housing in advance of crystal growth below,in the bestirred lower melt, to provide a more uniform radialcomposition both in said melt and crystal while k) replenishing saidlower melt from the upper reservoir melt through said annular spaces.